Question

In snail shells the direction of shell coiling is controlled by a maternal effect gene. Two alleles, a dominant one, D, causes dextral (right handed) coiling, and a recessive one, d, causing sinistral (left hand) coiling.

Given three different female snails of genotypes DD, Dd, and dd, crossed with dd males.

What are the phenotypes of the offspring of each

DD females:

Dd females:

dd females:

What are the phenotypes of these females. Explain, based on the probable parental (both maternal and paternal for each female. (Hint: you may not be able to give definitive answers for each. If so explain why in terms of the possible parents.)

DD female:

Dd female:

dd female:

Answer 1

The answer is explained below:

Step-by-step explanation:

• A maternal effect gene is the one which shows the same phenotype as that of the mother irrespective of the phenotype of the father.

• According to the question,

1. D is the dominant alelle, and show right-handed coil phenotype.
2. d is the recessive alelle, and show left-handed coil phenotype.

• As we do not know the phenotypes of the mothers of the DD, Dd, or dd females or the dd male taken to be crossed with each other, let us consider that the DD, Dd and dd female show a right hand coil phenotype and the dd male show a left hand coil phenotype.

• When DD female (right hand coil phenotype) is crossed with dd males (left hand coil phenotype),

D D

d Dd Dd

(R) (R)

d Dd Dd

(R) (R)

• All the offspring will have the Dd genotype with the (R), that is, right-hand coil phenotype.
• When Dd female (right hand coil phenotype) is crossed with dd males (left hand coil phenotype),

D d

d Dd dd

(R) (R)

d Dd dd

(R) (R)

• The genotypic ratio is, Dd : dd = 1 : 1.
• All the offspring have the dominant right hand coil phenotype, represented by (R), as this is a maternal effect gene and the phenotype of the mother is shown in all its offspring irrespective of their genotype.
• When dd female (right hand coil phenotype) is crossed with dd males (left hand coil phenotype),

d d

d dd dd

(R) (R)

d dd dd

(R) (R)

• All the offspring will have the dd genotype.
• All the offspring have the dominant right hand coil phenotype, represented by (R), as this is a maternal effect gene and the phenotype of the mother is shown in all its offspring irrespective of their genotype.